This invention relates to dispensers for flexible strand materials, and particularly to cassettes that deliver modeling material in the form of a continuous filament to a machine that forms three-dimensional objects by extruding modeling material in a flowable state in three dimensions with respect to a base.
Extrusion-based layered manufacturing machines build up three-dimensional objects or models by extruding solidifiable modeling material from an extrusion head in a predetermined pattern, based upon design data provided from a computer aided design (CAD) system. A feedstock of either a liquid or solid modeling material is supplied to the extrusion head. One technique is to supply modeling material in the form of a filament strand. Where the feedstock of modeling material is in solid form, a liquifier brings the feedstock to a flowable temperature for deposition. The modeling material is extruded layer-by-layer in areas defined from the CAD model. The material being extruded fuses to previously deposited material and solidifies to form a three-dimensional object resembling the CAD model.
Examples of extrusion-based apparatus and methods for making three-dimensional objects are described in Valavaara U.S. Pat. No. 4,749,347, Crump U.S. Pat. No. 5,121,329, Crump U.S. Pat. No. 5,340,433, Crump et al. U.S. Pat. No. 5,503,785, Danforth, et al. U.S. Pat. No. 5,900,207, Batchelder, et al. U.S. Pat. No. 5,764,521, Dahlin, et al. U.S. Pat. No. 6,022,207, Stuffle et al. U.S. Pat. No. 6,067,480, Batchelder, et al. U.S. Pat. No. 6,085,957, and PCT Publication No. WO 00/78519 A1, all of which are assigned to Stratasys, Inc., the assignee of the present invention.
One technique for supplying modeling material to a three-dimensional modeling machine provides modeling filament in an enclosed cassette. This technique is disclosed in Published U.S. patent application No. 20010030383, hereby incorporated by reference number. The cassette contains spooled filament which is advanced to the machine through an exit orifice of the cassette. Use of the cassette permits automatic loading of the filament. Also, the cassette can be made air tight and sealed so that the filament can be kept in a dry environment. As many modeling materials are moisture sensitive (i.e. high-temperature thermoplastics), maintaining dryness of the filament is in some cases essential for building accurate, robust models.
In the prior art cassettes such as disclosed in Published U.S. Application No. 20010030383, the spool can rotate inside of the cassette during transport and handling. This inadvertent rotation has been discovered to cause the filament to unravel, which sometimes results in tangling or breakage. In such a case, the cassette fails to properly deliver filament to a modeling machine. Further, even without unacceptable unwinding, it has been discovered that the cassettes sometimes fail due to imperfections in the filament wrap. During production, filament is wound tightly onto the spool. Imperfections in the wrap have been found to result in snagging of the filament as it unwinds.
In the '383 patent publication, a one-way ratchet spring device 222 is disclosed for preventing backwards movement of the filament. This device was found to successfully prevent backward rotation but it did not prevent forward rotation. Rather than decrease the failure rate, this resulted in increased tangling as the spool was permitted to rotate forward thereby unwinding filament, but could not likewise rotate backwards to tighten the wrap.
In an attempt to curtail the inadvertent forward and backward rotation of spools in the filament cassettes, a piece of adhesive-backed polyurethane foam was adhered to an interior wall of the cassette, in contact with the spool. The foam added friction so as to curtail the free rotation of the spool. The foam approach was found, however, to be unsatisfactory. While the foam did curtail rotation, it did not completely eliminate it. Even the slightest bit of rotation of the spool during shipping was demonstrated to cause a snag within the can, and even the slightest snag was fatal. Further, the foam caused a new set of filament delivery problems. In some cases, the cassettes failed because the friction on the spool was too great. Too much friction would stall out filament drive motors on the modeling machines. The amount of friction added by the foam was difficult to control and resulted in large friction variations amongst the cassettes. Other problems occurred as well. Sometimes the adhesive did not hold, causing the foam to detach from the cassette and the machine to stall due to increased friction. The foam had a tendency to crumble, and foam particles could make their way into the filament load path where they could jam or plug the modeling machine. The foam was also difficult to apply and to remove from the cassettes.
It would be desirable to increase the reliability of filament delivery by filament cassettes.